The present invention relates to the reading of information recorded in an information medium. More specifically, the present invention relates to a method of reading information utilizing a scanning tunneling microscope in conjunction with an information layer comprised of a fluorescent material.
Scanning tunneling microscopy is a fairly recent development, with Gerd Binnig and Heinrich Rohrer having received the 1986 Nobel Prize in physics for its development. While the early designs of this microscope were both cumbersome and delicate, recent improvements have led to scanning tunneling microscope (STM) units which are both compact and robust.
An STM unit is generally comprised of a piezoelectric positioning device that is capable of rastering a sharp metallic tip with sub-nanometer resolution across a sample of interest. The piezoelectric positioner can take many shapes and is often comprised of a set of three orthogonal piezoelectric bars or a single piezoelectric tube which has been sectioned into four quadrants. In addition to moving the tip across the sample, the piezoelectric positioning device must also maintain a constant distance between the tip and sample. This feature of all STMs requires a very sensitive height detector.
In an STM, the height detector relies on the quantum mechanical nature of the tunneling current that flows between the tip and sample. In order to set up a tunneling current, the tip must be about 0.5 nm above the sample. It is well established that the tunneling current varies exponentially with distance such that a change in tip-to-sample distance of 0.1 nm causes about a factor of ten change in tunneling current. This exquisite sensitivity of the tunneling current to the tip-sample separation is used as a feedback signal to the piezoelectric positioner, thus allowing the tip-to-sample separation to be held constant to better than 0.01 nm.
A suitably designed STM unit can be quite small and STMs as small as one centimeter in diameter have been built. The piezoelectric positioner and tip assembly must be carefully isolated from vibrations and this is often accomplished by suspending the STM with spring-like supports. Often, two or three levels of vibration isolation are incorporated. However, by making the physical size of the instrument small and designing the piezoelectric positioner into a highly symmetric holder, the severe requirements on vibration isolation can be reduced to a manageable and easily achievable level.
The second requirement for a stable instrument is a high degree of temperature compensation. This is desirable because even a temperature gradient as small as 0.01K can cause unacceptably large drifts due to the uncontrolled thermal expansion of the piezoelectric positioner. Thus by carefully balancing the thermal expansion of the structural elements of the instrument against the expansion properties of the piezoelectric material, a high degree of temperature compensation can be automatically achieved, resulting in a constant tip-to-sample distance even while operating in an ambient air environment.
Based on recent developments in the design of STM instruments, it is anticipated that smaller, more stable, and more compact scanning assemblies will be developed within the foreseeable future. These STMs will routinely allow experiments at a length scale unimaginable only a few years ago. Of considerable current interest is the ability of the STM to modify in a controlled way the properties of matter at the nanometer length scale. In fact, the physical principles underlying the STM have already provided a means to alter and fabricate structures at the atomic level. Further background and detail with regard to the development and operation of a scanning tunneling microscope is also found in the paper "Scanning Tunneling Microscopy--From Birth to Adolescence", by Gerd Binnig and Heinrich Rohrer, reprinted in Rev. Mod. Phys., Volume 59, No. 3, Part I, July 1987.
Electron tunneling is the phenomenon that underlies the operation of the scanning tunneling microscope. An electron cloud generally occupies a space between the surface of the sample and the needle tip used in the microscope. The cloud is a consequence of the indeterminacy of the electron's location (a result of its wavelike properties). Because the electron is "smeared out", there is a probability that it can lie beyond the surface boundary of a conductor. The density of the electron cloud decreases exponentially with distance. A voltage-induced flow of electrons through the cloud is therefore extremely sensitive to the distance between the surface of the sample, and the scanning needle tip.
To scan the surface, the tip of the needle is pushed toward the sample until the electron clouds of each gently touch. The application of a voltage between the tip and the sample causes electrons to flow through a narrow channel in the electron clouds. This flow is called the tunneling current. A change in the distance between the scanning needle tip and the surface of the sample by an amount equal to the diameter of a single atom causes a tunneling current to change by a factor as much as 1,000. Thus, extremely precise measurements of the vertical positions of the atoms on the sample surface may be obtained.
Due to the extraordinary sensitivity of the scanning tunneling microscope, it has become an important tool in surface science and physics in general. Its primary use has been to obtain atomic-resolution images of surfaces. However, efforts have also been used to manipulate materials as well as image them. See, for example, "Molecular Manipulation Using a Tunneling Microscope", by J. S. Foster, J. E. Frommer and J. C. Arnett, Nature, Volume 331, Jan. 28, 1988, Page 324; and, "Atomic-Scale Engineering" by J. B. Pethica, Nature. Volume 331, Jan. 28, 1988, Page 301.
A scanning tunneling microscope has also been applied in lithography. For example, lithography with a scanning tunneling microscope has been demonstrated by fabricating submicron lines using a "contamination process" and a Langmuir-Blodgett film as resists. Lithography using metal halide films and polymethylmethacrylate films have also been studied. For example, see McCord and Pease, J. Vac. Sci. Technol. B5(1), January/February 1987, Page 430 and J. Vac. Sci. Technol. B, Volume 6, No. 1, January/February 1988, Page 293; and, Y. Z. Li et al., Appl. Phys. Lett., Vol. 54, 1424 (1989).
The high resolution available through the application of a scanning tunneling microscope is certainly a most desirable attribute, and is indeed necessary if information were ever recorded at densities approaching from 10.sup.12 to 10.sup.14 bits/cm.sup.2. The spatial resolution realized by use of the scanning tunneling microscope would certainly allow such high density information to be read efficiently and accurately. However, the present reading capabilities of a scanning tunneling microscope are too slow for any practical application in mass data storage. This inherent slowness of the device is due to the limited practical rate at which one could successfully modulate the voltage of the scanning tunneling microscope tip. If a method were found which could successfully utilize the resolution of a scanning tunneling microscope, without necessarily suffering from the inherent slowness in voltage modulation, a most impressive system would be obtained.
Accordingly, it is an object of the present invention to provide a method which employs a scanning tunneling microscope in the reading of information available on an information medium.
It is another object of the present invention to employ a novel method of reading information utilizing a scanning tunneling microscope which does not suffer from the inherent slowness of voltage modulation.
It is still a further object of the present invention to provide a method which permits the reading of information with high spatial resolution approaching information densities of from 10.sup.12 to 10.sup.14 bits/cm.sup.2.
These and other objects of the present invention will become apparent upon a review of the following specification and the claims appended thereto.